In knee joint replacement surgery, a surgeon typically affixes two prosthetic components to the patient's bone structure; a first to the patient's femur and a second to the patient's tibia. These components are typically known as the femoral component and the tibial component respectively.
The femoral component is placed on a patient's distal femur after appropriate resection. One common type of femoral component features a J-shaped cross section. The femoral component is usually metallic, having a highly polished outer condylar articulating surface.
A common type of tibial component uses a tray or plate that generally conforms to the patient's resected proximal tibia. The tibial component also usually includes a stem which extends generally perpendicular to the plate in order to extend into a surgically formed opening in the patient's intramedullary canal.
A plastic or polymeric (often high density polyethylene or HDPE) insert or bearing fits between the plate of the tibial component and the femoral component. This insert or bearing provides a surface against which the femoral component condylar portion articulates (moves in gross motion corresponding generally to the motion of the femur relative to the tibia).
In so-called mobile bearing knee prostheses, the bearing also engages in motion relative to the plate. Such motion can be translational and/or rotational sliding motion relative to the plate. In other types of mobile bearing knee prostheses, the bearing can engage in other types of motion relative to the plate and/or femoral component.
Portions of the tibial and femoral components may be at least partially porous coated, or they may be non-porous coated. Porous coating surfaces of implant components promote bone ingrowth, which supplants the need for some or all of the cement typically employed with non-porous coated implant components.
Accurately positioning and fitting the prosthetic components is of paramount interest to the surgeon and to the patient for a number of reasons. Each patient has a different bone structure and geometry, as a static matter. Dynamically, motion of the tibia relative to the femur about every axis varies from one patient to the next. Even though the surgeon uses various imaging techniques and palpation to study a particular patient's anatomy, she nevertheless gains considerable additional information required to fit the prosthetic components after the knee has been surgically exposed and she begins the surgery.
This diversity of knee structure, geometry and dynamics compels most suppliers of prosthetic components to offer a wide range of prosthetic options for knee replacement surgeries. These include, for instance, femoral and tibial components for primary surgeries as well as revision surgeries, porous coated and non-porous coated components, various sizes of stems for various tibial component plates, various femoral component interfaces for primary and revision cases, and prostheses which feature mobile bearings as well as those which do not. The present invention is intended for use with any of these options.
So-called trial prostheses are conventional for, among other things, trying the fit of prosthesis or implant components to respective portions of the joint. After shaping the femur and the tibia, the surgeon may temporarily fit trial components instead of the actual prosthetic components to the femur and/or tibia respectively. This enables the surgeon to test the fit of the components to the femur and tibia and to test their performance both statically and dynamically throughout a desired range of motion. Use of trial prosthetics instead of the actual implants also allows the surgeon to perform this testing and achieve a more perfect fit and a more accurate performance of the actual component without introducing a number of “new” actual prosthetic components into the surgical field.
Using actual prosthetic components for this fitting procedure is undesirable for a number of obvious reasons. For example, the trial prosthetic components allow the surgeon to position, move, and fit components while trying various sizes and, if desired, while modifying bone structure, without imparting wear and tear on actual prosthetic components, upon which destruction could have adverse long-term effects.
Modularity serves many interests in implant prosthetics as well as trial prosthetics. For instance, a particular tibial plate may accommodate a range of sizes and angles of stems which fit to the plate via a Morse taper. The surgeon thus has a range of options available without subjecting the patient or the organization to the expense of a separate entire tibial component corresponding to each option. Similarly, it is desirable in tibial trial prosthetics to employ the notion of modularity. Thus, it is preferable in many cases to offer a tibial trial component which features a base plate and a number of stems of varying lengths and perhaps varying angles which are adapted to couple or connect to the plate in order to correspond to a range of sizes and configurations of actual tibial components. U.S. Pat. Nos. 5,609,642, 5,683,469, 5,683,470, and 5,776,200 to Johnson, et al., which are incorporated herein by this reference, disclose tibial trial components which are modular in nature. These patents disclose, among other things, a tibial trial prosthesis which include a plate that attaches to a trial stem using a trial taper and a trial stem coupler. The taper receives the coupler and thereby the stem, and it fits to the plate via a component such as a bushing, which is inserted through the top or proximal surface of the plate.
Tibial plate design can impose constraints on how the stem trial is connected to the plate trial. Plates, both implant and trial, can be relatively thin, and they are often not totally flat or planar in structure. For example, many non-porous tibial plates, and their corresponding trial plates, include fins that extend from the distal surface into the tibia for rotational stability. Additionally, tibial plates which receive mobile bearings include a post such as a T-post which extends from the proximal surface of the plate in order to retain and restrain the mobile bearing to certain degrees of freedom and range of motion. The post structure and other structures on either surface of the plate can occupy the space on or within the plate through which a member for retaining the stem would be inserted through the proximal surface. Additionally, such a proximal surface-inserted member could interfere with insertion of devices for forming the tibia to receive the fins. It may also interfere with movement of the bearing or insert relative to the plate in a mobile bearing design. Accordingly, not all tibial trial components are well-suited for the sort of modular approach disclosed in the above-referenced Johnson, et al. patents.
In one commercial offering, for example, the tibial base plate of a mobile bearing system includes a T-post and a tapered stem. The non-porous tibial base plate also includes fins for rotational stability. A simple technique and instrumentation system is required to trial the tibial base plate and to prepare the proximal tibia for the stem and the fins. (Again, the present invention is not limited to use with tibial plates that include T-post and fins, but may be used with any plate.) The bone cannot be prepared for fins through the tibial trial plate when the position of the T-post interferes with current punching techniques.